Electric distribution equipment are often protected by fast response circuit interruption devices such as circuit breakers and contactors as controlled by protective relays such as described within U.S. patent application Docket No. 11RC-4923 entitled "A Hybrid Protective Relay Having Enhanced Contact Response Time" filed concurrently herewith.
Control signals to the relays are provided by means of communications buses such as described within U.S. patent application Ser. No. 08/628,533 filed Apr. 3, 1996 entitled "Dynamic Data Exchange Server Simulator".
Oftentimes the control signals are provided to the protective relays over standard current-carrying conductors that connect between the electric equipment and the relays. In this application, voltage signals are sent to the relays at different voltage levels depending upon the type of electric equipment involved. It is not uncommon to have operating voltages ranging from 48-250 volts, for example.
Prior input circuits employ high voltage type power transistors to insure that the transistors are voltage-rated to withstand the higher voltage signals. To handle a wide operating range, the circuits are made very sensitive. Leakage currents on open, high voltage circuits can cause false indications of the presence of an input signal.
The use of analog type voltage comparators improves the circuit response over a wide range of signal voltages. However, to provide programmable levels, individual, isolated reference voltage adjustments would be needed.
It would be desirable to provide programmable response to voltage signals over a range from low to high values without requiring a number of high voltage transistors, or manual adjustment of the input signals via a potentiometer or like device.
One purpose of the invention, accordingly, is to provide a simple electronic circuit for providing input to protective relays over a wide range of operating voltages by transferring the input voltage signals to proportionate frequency signals at a fixed voltage value and transferring the signals to a processor by means of a conventional 8-bit counter.